Don't let my wife see this......Let's break down the the main parts to buy.
Car(with complete "usable" interior)? Running and driving?
Engine/trans (In your possession or must buy?)
Wheels/Tires (In your possession or must buy?)
The 8 million other parts that will nickel and dime you to death
Unless you want a stripped out, no interior, racecar G-bodies have left the budget realm of builds unless you are lucky on Facebook/Craigslist or will settle with a more door.
Buying a finished car will always be cheaper. Building a G-body is about the love and not crying when you spend all that money for a car that would sell for half of what it cost to build if you don't write off your "free" labor.